1. Field of the Invention
This invention relates to removal of water from fibrous suspensions. More specifically, this invention relates to the treatment of exhaust air from a sludge drying system for producing granular particles or pellets to be used as fertilizer.
2. Description of the Prior Art
Sludge from a municipal wastewater treatment plant or a paper mill is mainly mechanically dewatered. This mechanically dewatered sludge is fed to a sludge drying plant which utilizes heated air in a rotating drying drum. The dried sludge is pelletized and can be either sold as fertilizer or disposed of as permitted by restricting regulations. The air discharged from the dryer, i.e., exhaust air, is cooled after separation from the sludge particles. A main portion of the exhaust air is recycled to the burner. The rest of the exhaust air is released into the atmosphere. If not further treated, the exhaust air released into the atmosphere will usually include unwanted organic particles which may accumulate during the drying process. This exhaust air may cause emission problems due to its odor and organic substance content which is being restricted to low concentrations.
It is known that such organic substances are inactivated at temperatures of above 800.degree. C. and this has lead to the use of afterburners for removal from the exhaust air. Such system is disclosed in VSA--Dokumentation, "Klarschlamm behandeln" by U. Keller (1981). A process using afterburners will require additional energy to burn the exhaust air and expensive equipment to be added to the system. Also, the amount of exhaust air created by the entire sludge drying process will increase considerably.
Examples of other sludge drying systems are disclosed in U.S. Pat. Nos.: 3,410,233 to Seiler; 3,963,471 to Hampton; 4,761,893 to Glorioso; 4,901,654 to Albertson et al; 4,926,764 to van den Broek; 4,953,478 to Glorioso; and 5,069,801 to Girovich, all of which are hereby incorporated herein by reference.
Such prior systems provide a wet scrubber and/or an afterburner for treating exhaust air, which require extensive modification to existing systems and have high operating costs.
A major cost factor of prior systems is a separate afterburner for burning the exhaust air containing the organic particles. The exhaust air is wet and loaded with particulates and other contaminants. Thus, the exhaust air is subjected to scrubbing and cooling to remove some of the particulates and water prior to the afterburner. This results in loss of heat from the exhaust air, which is reheated in the afterburner.
This invention addresses these problems in the art as well as other problems in the art which will be apparent to those skilled in the art once given this disclosure.